I work on https://www.blabladns.co - a free dynamic dns service with an easy api and many integrations (e.g. Slack, PagerDuty, Messenger, etc.).
Working Solo on it and its not profitably, because there is no paid plan yet. More premium features will cost and will be integrated soon.
Growth? Very slowly with one to two user per week. Probably because I dont do marketing at all (only the blog).